This invention is related with a process to produce sodium sulfate and magnesium hydroxide. Particularly, the invention is related with the precipitation of Mg++ as Mg(OH)2 with an alkali, which consumes and regenerates within an internal cycle of the same process; the precipitation of Na+ is carried out by adding a salt more soluble than Na2SO4 which it precipitates, and also, the said soluble salt is the source of Na+ and Mg++ from where the sodium sulfate and magnesium hydroxide is obtained.